On Sat, May 25, 2019 at 02:14:07PM -0400, Joel Fernandes wrote: > On Sat, May 25, 2019 at 08:50:35AM -0700, Paul E. McKenney wrote: > > On Sat, May 25, 2019 at 10:19:54AM -0400, Joel Fernandes wrote: > > > On Sat, May 25, 2019 at 07:08:26AM -0400, Steven Rostedt wrote: > > > > On Sat, 25 May 2019 04:14:44 -0400 > > > > Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > I guess the difference between the _raw_notrace and just _raw variants > > > > > > is that _notrace ones do a rcu_check_sparse(). Don't we want to keep > > > > > > that check? > > > > > > > > > > This is true. > > > > > > > > > > Since the users of _raw_notrace are very few, is it worth keeping this API > > > > > just for sparse checking? The API naming is also confusing. I was expecting > > > > > _raw_notrace to do fewer checks than _raw, instead of more. Honestly, I just > > > > > want to nuke _raw_notrace as done in this series and later we can introduce a > > > > > sparse checking version of _raw if need-be. The other option could be to > > > > > always do sparse checking for _raw however that used to be the case and got > > > > > changed in http://lists.infradead.org/pipermail/linux-afs/2016-July/001016.html > > > > > > > > What if we just rename _raw to _raw_nocheck, and _raw_notrace to _raw ? > > > > > > That would also mean changing 160 usages of _raw to _raw_nocheck in the > > > kernel :-/. > > > > > > The tracing usage of _raw_notrace is only like 2 or 3 users. Can we just call > > > rcu_check_sparse directly in the calling code for those and eliminate the APIs? > > > > > > I wonder what Paul thinks about the matter as well. > > > > My thought is that it is likely that a goodly number of the current uses > > of _raw should really be some form of _check, with lockdep expressions > > spelled out. Not that working out what exactly those lockdep expressions > > should be is necessarily a trivial undertaking. ;-) > > Yes, currently where I am a bit stuck is the rcu_dereference_raw() > cannot possibly know what SRCU domain it is under, so lockdep cannot check if > an SRCU lock is held without the user also passing along the SRCU domain. I > am trying to change lockdep to see if it can check if *any* srcu domain lock > is held (regardless of which one) and complain if none are. This is at least > better than no check at all. > > However, I think it gets tricky for mutexes. If you have something like: > mutex_lock(some_mutex); > p = rcu_dereference_raw(gp); > mutex_unlock(some_mutex); > > This might be a perfectly valid invocation of _raw, however my checks (patch > is still cooking) trigger a lockdep warning becase _raw cannot know that this > is Ok. lockdep thinks it is not in a reader section. This then gets into the > territory of a new rcu_derference_raw_protected(gp, assert_held(some_mutex)) > which sucks because its yet another API. To circumvent this issue, can we > just have callers of rcu_dereference_raw ensure that they call > rcu_read_lock() if they are protecting dereferences by a mutex? That would > make things a lot easier and also may be Ok since rcu_read_lock is quite > cheap. Why not just rcu_dereference_protected(lockdep_is_held(some_mutex))? The API is already there, and no need for spurious readers. > > That aside, if we are going to change the name of an API that is > > used 160 places throughout the tree, we would need to have a pretty > > good justification. Without such a justification, it will just look > > like pointless churn to the various developers and maintainers on the > > receiving end of the patches. > > Actually, the API name change is not something I want to do, it is Steven > suggestion. My suggestion is let us just delete _raw_notrace and just use the > _raw API for tracing, since _raw doesn't do any tracing anyway. Steve pointed > that _raw_notrace does sparse checking unlike _raw, but I think that isn't an > issue since _raw doesn't do such checking at the moment anyway.. (if possible > check my cover letter again for details/motivation of this series). Understood, but regardless of who suggested it, if we are to go through with it, good justification will be required. ;-) Thanx, Paul > thanks! > > - Joel > > > Thanx, Paul > > > > > thanks, Steven! > > > > > >
